1. Field of the Invention
The present invention relates to a method for producing a ceramic honeycomb filter which is used for removing fine particles in diesel engine exhaust gas or for chemical industries, or a ceramic honeycomb structural body for heat exchangers. More particularly, this invention relates to a method for sealing open ends of throughholes of the ceramic honeycomb structural body.
2. Description of the Prior Art
Regarding a method for sealing open ends of throughholes of the ceramic honeycomb structural body, there is known a method which comprises steps of attaching a film or mask on an open end surface of a ceramic honeycomb structural body, and introducing a ceramic sealing material into throughholes of the ceramic honeycomb structural body through apertures provided at selected positions of the film or mask, to seal an end portion of the throughholes of the ceramic honeycomb structural body (U.S. Pat. Nos. 4,293,357 and 4,559,193).
In U.S. Pat. No. 4,293,357, as the ceramic sealing materials, those having dilatancy are used, and U.S. Pat. No. 4,559,193 discloses a method comprising steps of dipping a honeycomb body in a slurry containing a sealing material and passing a pasty sealing material green body into throughholes of the honeycomb body.
However, the use, of a dilatant sealing material has drawbacks in that a tight seal cannot be provided between partition walls defining the throughholes and the sealing material, because of a smaller diameter of apertures bored through the film as compared with the diameter of the throughholes of the ceramic honeycomb structural body, or the sealing material bridging the apertures of the film due to the dilatancy, which hinders a smooth introduction of the sealing material into the throughholes.
It is more effective for ensuring a tight sealing between partition walls and sealing layer to use a viscous slurry. However, the use of such a slurry has drawbacks in that an open pore is prone to be formed in the center portion of the sealing layer. In particular, for sealing of a porous ceramic honeycomb structural body, the drawbacks become more conspicuous because the sealing material penetrates through the pores of the porous ceramic body to draw the sealing layer towards the partition walls.
Therefore, the use of two phases of sealing materials, i.e., a viscous material and a dilatant material, has been proposed to ensure a tight sealing without forming the central hole. However,the use of two phases of sealing material requires two sealing steps, which is not suitable for mass production.